Simon and Garfunkel's planned tour of North America has been "postponed indefinitely" because Art Garfunkel has a condition affecting his voice.
He is recovering from a vocal paresis which "inhibits the duo from performing shows" to their normal standard, a statement on their website said.
The tour had already been postponed from April to July.
The stars, who performed together in the US earlier this month for a TV special, are offering refunds.
On 10 June, they performed Mrs Robinson, which featured in 1967's The Graduate, at an event honouring the film's director, Mike Nichols.
But Garfunkel would not be fit for a full tour, the duo explained.
"According to his doctors, Art is expected to make a full recovery. However, they cannot predict an exact timeline," a statement said.
Garfunkel added: "I do feel bad about disrupting so many people's plans but, as I continue to mend, I can't yet bring my A-game to a tour, and I would not perform for you with anything less."
The 68-year-old singer revealed the duo's plans to tour again in an interview with the BBC News website in February 2009.
He spoke of the plans five days after making a surprise appearance at a Paul Simon concert in New York, where the pair performed hits like The Sounds of Silence, The Boxer and Old Friends.
He told BBC News they had received "a hell of a knockout of a reaction".
He added: "I said to the audience, 'you don't have a right to expect - and we didn't expect - that the interest in us would last and you would still care right to today and I'm touched'."
Simon and Garfunkel, who last toured together in 2003 and 2004, have famously fallen out on a number of occasions since they first made music together as school friends in New York in the 1950s.